In the recent drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The photolithography which is currently on widespread use in the art is approaching the essential limit of resolution determined by the wavelength of a light source. As the light source used in the lithography for resist pattern formation, g-line (436 nm) or i-line (365 nm) from a mercury lamp was widely used in 1980's. Reducing the wavelength of exposure light was believed effective as the means for further reducing the feature size. For the mass production process of 64 MB dynamic random access memories (DRAM, processing feature size 0.25 μm or less) in 1990's and later ones, the exposure light source of i-line (365 nm) was replaced by a KrF excimer laser having a shorter wavelength of 248 nm. However, for the fabrication of DRAM with a degree of integration of 256 MB and 1 GB or more requiring a finer patterning technology (processing feature size 0.2 μm or less), a shorter wavelength light source was required. Over a decade, photolithography using ArF excimer laser light (193 nm) has been under active investigation. It was expected at the initial that the ArF lithography would be applied to the fabrication of 180-nm node devices. However, the KrF excimer lithography survived to the mass-scale fabrication of 130-nm node devices. So, the full application of ArF lithography started from the 90-nm node. The ArF lithography combined with a lens having an increased numerical aperture (NA) of 0.9 is considered to comply with 65-nm node devices. For the next 45-nm node devices which required an advancement to reduce the wavelength of exposure light, the F2 lithography of 157 nm wavelength became a candidate. However, for the reasons that the projection lens uses a large amount of expensive CaF2 single crystal, the scanner thus becomes expensive, hard pellicles are introduced due to the extremely low durability of soft pellicles, the optical system must be accordingly altered, and the etch resistance of resist is low; the development of F2 lithography was stopped and instead, the ArF immersion lithography was introduced.
In the ArF immersion lithography, the space between the projection lens and the wafer is filled with water having a refractive index of 1.44. The partial fill system is compliant with high-speed scanning and when combined with a lens having a NA of 1.3, enables mass production of 45-nm node devices.
One candidate for the 32-nm node lithography is lithography using extreme ultraviolet (EUV) radiation with wavelength 13.5 nm. The EUV lithography has many accumulative problems to be overcome, including increased laser output, increased sensitivity, increased resolution and minimized line-edge roughness (LER or LWR) of resist film, defect-free MoSi laminate mask, reduced aberration of reflection mirror, and the like.
Another candidate for the 32-nm node lithography is high refractive index liquid immersion lithography. The development of this technology was stopped because LUAG, a high refractive index lens candidate had a low transmittance and the refractive index of liquid did not reach the goal of 1.8.
The process that now draws attention under the above-discussed circumstances is a double patterning process involving a first set of exposure and development to form a first pattern and a second set of exposure and development to form a pattern between the first pattern features. A number of double patterning processes are proposed. One exemplary process involves a first set of exposure and development to form a photoresist pattern having lines and spaces at intervals of 1:3, processing the underlying layer of hard mask by dry etching, applying another layer of hard mask thereon, a second set of exposure and development of a photoresist film to form a line pattern in the spaces of the first exposure, and processing the hard mask by dry etching, thereby forming a line-and-space pattern at a half pitch of the first pattern. An alternative process involves a first set of exposure and development to form a photoresist pattern having spaces and lines at intervals of 1:3, processing the underlying layer of hard mask by dry etching, applying a photoresist layer thereon, a second set of exposure and development to form a second space pattern on the remaining hard mask portion, and processing the hard mask by dry etching. In either process, the hard mask is processed by two dry etchings.
Patent Document 1 discloses a process involving forming a first pattern of a positive resist material through exposure and alkaline development, rendering the first pattern insoluble in organic solvent and alkaline developer with the aid of acid and heat, coating another positive resist material thereon, and forming a second pattern through exposure and alkaline development. Patent Document 2 discloses a process involving forming a first pattern of a positive resist material through exposure and alkaline development, rendering the first pattern insoluble in organic solvent and alkaline developer with the aid of light irradiation and heat, coating another positive resist material thereon, and forming a second pattern through exposure and alkaline development. These processes belong to a double patterning process involving the steps of insolubilizing a first positive resist pattern and combining it with a second positive pattern.
As compared with the line pattern, the trench or hole pattern is difficult to reduce its size. If an attempt is made to form fine holes according to the prior art method by combining a positive resist film with a hole pattern mask and effecting under-exposure, the exposure margin is extremely narrowed. It is then proposed to form holes of larger size and shrink the developed holes by thermal flow, RELACS® or other techniques. However, there is a substantial difference between the pattern size as developed and the pattern size as shrunk, giving rise to the problem that a greater shrinkage leads to a lower control accuracy. With the hole shrinking method, the hole size can be shrunk, but the pitch cannot be narrowed.
Holes can be shrunk using the direct self assembly (DSA) technology. A typical DSA material is a styrene-methacrylate block copolymer. The DSA technology can shrink a hole pattern if holes are of true circle. When the DSA technology is applied to elongated holes or trench patterns, juxtaposed holes are formed. It is a drawback that shrinkage occurs while the original shape is deformed.
Recently a highlight is put on the organic solvent development again. It would be desirable if a very fine hole pattern, which is not achievable with the positive tone, is resolvable through negative tone exposure. To this end, a positive resist composition featuring a high resolution is subjected to organic solvent development to form a negative pattern. An attempt to double a resolution by combining two developments, alkali development and organic solvent development is under study.
As the ArF resist composition for negative tone development with organic solvent, positive ArF resist compositions of the prior art design may be used. Such pattern forming process is described in Patent Document 3.
An attempt to form elongated holes via negative development results in elliptic shape. This is due to the influence of optical interference. The elliptic hole pattern is undesired when it is intended to form a rectangular hole pattern.
Non-Patent Document 1 proposes a process for reducing the pitch to one half by heating a first positive resist pattern for insolubilizing it in organic solvent and alkaline developer, coating a second resist material, and forming a second positive pattern between the first positive pattern features. If the second pattern is orthogonal to the first pattern, a hole pattern can be formed. If the pitch is changed between the first and second lines, rectangular holes can be formed. However, since both the first and second steps use positive resist materials, formation of a narrow trench pattern is disadvantageous from the standpoint of optical contrast.
Patent Document 4 proposes an image reversal technology involving forming a positive resist pattern of positive resist material via alkaline development, heating the positive resist pattern for insolubilizing it in organic solvent while maintaining alkaline development capability, coating a film of low alkaline solubility, and effecting development to dissolve only the surface layer of the low alkaline solubility film while leaving the majority of film, and to dissolve the positive resist pattern in alkaline developer. Since the positive resist pattern is merely insolubilized in only organic solvent, this technology does not need a high degree of insolubilization and thus prevents deformation of the positive pattern.